In coal-fired boilers, ash is generally removed from the furnace through a bottom opening therein. The ash falls into an ash hopper filled with water, from which the ash is intermittently removed. Water is constantly added to the ash hopper to replace water that evaporates and escapes up into the furnace, and also water removed from the hopper along with the ash. In addition, water is constantly added, and some removed, to keep the temperature of the water low enough so that the ash is thermally shocked when it hits the water. This causes large chunks of ash to disintegrate or splinter, when contacting the water, minimizing the formation of large clinkers, which is undesirable. In the past, there was little concern shown for the amount of water used, and a steady, high-flow rate of water was continuously passed into and out of the ash hopper to keep it at a low temperature. This high-flow rate was maintained under all operating conditions of the boiler. The water was usually taken from a lake or river, and the overflow from the ash hopper was discharged to a stagnant storage pond. Now that water pollution has become of vital concern, the above type of operation is highly undesirable.